Prevention of the driven wheels from an excessive slip or spinning on a road surface is extremely useful for the sake of safety as well as for an effective provision with the driving force of a vehicle. The prevention may be achieved by decreasing a torque to be transmitted to the driven wheels--a torque being the cause of the slip or spinning.
Slip control systems of this type are disclosed in Japanese Patent Early Publication (Laid-Open) No. 16,948/1983 (corresponding to U.S. Pat. No. 4,484,280) and Japanese Patent Early Publication (Laid-Open) No. 56,662/1985 (corresponding to U.S. Pat. No. 4,583,611). The systems disclosed in these two prior patent applications involve, in each case, a technique using the application of a braking force by the brake to the driven wheels and a reduction in the output torque of the engine itself in order to decrease the torque to be transmitted to the driven wheels. More specifically, Japanese Patent Early Publication No. 16,948/1983 discloses a system in which the braking of the driven wheels only is carried out when a slip of the driven wheels is small, on the one hand, and the output torque of the engine is caused to be decreased, in addition to the braking of the driven wheels, when the slip of the driven wheels becomes large, on the other hand. Japanese Patent Early Publication No. 56,662/1985 discloses a system in which, when a slip of only one side of the left and right driven wheels is larger than that of the other side thereof, the one side thereof alone is braked and, when slips of both the left and right driven wheels are large, the both sides of the driven wheels are braked and further the output torque by the engine is caused to be reduced. The prior art systems as disclosed in the above patent applications are such that the braking of the driven wheels by the brake is primarily utilized and the reduction in the output torque of the engine is secondarily utilized.
Both the slip control by regulating the braking force and the slip control by controlling the output torque, as have been described above, have each advantages and disadvantages. The slip control using the braking force, on the one hand, is superior in response, while it is likely to cause a shock, leading to the impairment of the driving feeling. Furthermore, it is disadvantageous in terms of an effective use of the energy or a durability of the brake. The slip control regulating the output torque, on the other hand, provides a smooth variation in the torque to thereby secure a favorable driving feeling and outputs no useless torque to thereby improve an energy efficiency, while it is disadvantageous in terms of responsiveness.
From the foregoing, it is found extremely preferable to carry out both the slip control using the braking force and the slip control by the reduction in the output torque from the engine, when an amount of the slip or spinning of at least the driven wheels is large. Furthermore, the use of the two slip controls is preferable to secure a quick response, a good driving feeling, a high energy efficiency and a high durability of the brake in a favorably balanced manner.
In conducting both the slip control using the braking force and the slip control by regulating the output torque, however, the issue is to determine what ratio a reduced torque submitted to the driven wheels for the slip control should account for in a control ratio of a torque amount reduced by a reduction in the output torque to a torque amount reduced by the braking force. If such a control ratio is set constant as an optimum value for a certain driving condition, it is not always satisfactory for other driving conditions. This issue should be taken into considerations in instances where a vehicle is stuck in the mud or the like or where an operator prefers.
When an operator requires acceleration, a quick response is desirable while the driven wheels are prevented from slipping or spinning excessively by the slip control. In particular, in instances where the above control ratio is set from the viewpoint of lessening a frequency of the use of the brake, the output torque of the engine as a power source is caused to be reduced to a large extent. Accordingly, the issue in this instance is how to prevent the impairment of the acceleration accompanied with the reduction in the output torque, viz., how to prevent a delay in an increase in the output torque.
There are instances where the vehicle cannot move with the driven wheel or wheels stuck in the mud or the like. In these cases, an operator should operate the vehicle slowly by using a clutch in a skillful manner while controlling the slip of the driven wheel or wheels. In order to make the start-up of the vehicle in a stuck manner, it is also possible to use the slip control of the driven wheel or wheels while being stuck. In this case, the issue is how to match phenomena caused to occur by the slip control with a feeling of the operator, while the operator uses the clutch or accelerator skillfully as have been described above, in such a manner that the operator holds no undesirable feeling. When the vehicle is stuck, an amount of the slip or spinning of the driven wheel becomes considerably large so that both the slip control by the braking force and the slip control by reducing the output torque are used.
Further, if the driving feeling during the slip control could be set so as to meet an operator's preference, particularly a skill of the operator, it is extremely useful from the practical point of view.